Engine systems may be configured with boosting devices, such as turbochargers, for providing a boosted air charge and improving peak power outputs. Turbochargers include a turbine in the engine's exhaust path that harnesses some of the available exhaust energy to drive an attached compressor, placed upstream of the engine that forces more flow through the intake manifold. As the turbine spins up, the exhaust pressure rises, reducing the outlet flow from the combustion process, until the compressor sufficiently boosts the intake manifold pressure to overcome the back pressure. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits.
Boost pressure may be controlled during engine operation so that the advantages of a boosted operation can be balanced against potential issues associated with too much or too little boost. For example, in a diesel engine, too little boost can result in excessive particulate matter (PM) formation and limited drivability. As another example, too much boost can result in noise vibration harshness (NVH) and limited use of exhaust gas recirculation (EGR) due to higher intake manifold pressures, where the reduction in inert exhaust gas usage results in higher combustion temperatures and increased exhaust NOx formation. In one example, boost pressure may be controlled by adjusting the geometry of a variable geometry turbine of the turbocharger, such as by varying a blade angle of a variable geometry turbine (VGT).
Boost pressure control systems may be periodically diagnosed, such as via the execution of an on-board diagnostic (OBD) monitor. The monitor is expected to provide a definitive fault/no fault status.
In regard to monitor methods, one example approach is shown by Romzek in U.S. Pat. No. 6,457,461. Therein, degradation in boost control via a VGT is inferred based on a difference between the magnitudes of an actual EGR flow rate related to an expected EGR flow rate during boosted engine operation, the expected EGR flow rate determined by modeling. Still other approaches rely on the output of one or more of a boost pressure sensor, a manifold pressure sensor, and a throttle position, for example.
However the inventors herein have identified potential issues with such approaches. As one example, the approaches rely on complex and computation intensive modeling. The required computation power may not be available in a resource limited vehicle system. As another example, the monitor may be intrusively run wherein a significant change in boost demand is deliberately generated and then the magnitude of the boost response is measured. The intrusive test stimulus may disrupt vehicle drivability and emissions. If the test is performed non-intrusively, where vehicle operation is not disrupted, the minimum conditions required to determine, with high confidence, that a fault is present may not be met. Overall, it may be difficult to provide both adequate detection and minimal disruption. As still another example, in each of the above approaches, irrespective of which sensor is used, the magnitude of change in sensor output, as well a rate of change in the sensor output, may be affected by vehicle driving conditions, such as whether the vehicle operator is mildly or severely accelerating the vehicle, frequency of application and shift between accelerator and brake pedal usage, etc. As a result, due to an error in modeling the expected output, there may be conditions where a significant difference between expected and actual sensor output is incorrectly attributed to a degraded boost response, or an insignificant difference is incorrectly associated with an un-degraded boost response.
The inventors herein have recognized that boost pressure control includes a feedback loop, and that the time taken by the feedback loop to respond to a pressure deviation may be leveraged as a monitor because the feedback must constantly correct for command changes and outside disturbances making the feedback sensitive to boost control response degradation. Specifically, instead of relying on carefully controlled open loop command and time-to-response tests which require intrusive operation, the feedback control response time can be learned in a non-intrusive manner, and used to assess boost control. Thus in one example, the issues described above may be addressed by a method for a boosted engine comprising, while operating an engine with closed-loop boost control, indicating degradation of a boost pressure control system based on a measured deviation between expected boost pressure and actual boost pressure that frequently occurs during a drive cycle, where the difference is large enough that the control response should be strongly elevated. A duration over which the deviation persists may be monitored. A slow response is determined is the duration is longer than a threshold duration. The slow response detected here may occur for various physical reasons, such as manifold leaks and/or slow actuation, so the detection is not limited to actuator only degradation. In this way, since both command changes and/or disturbances offer the opportunity to observe control error response, boost control may be diagnosed passively, as long as some check is made to confirm adequate drive stimulus has occurred because the monitor must eventually declare a fault/no fault condition for at least one third of all qualified drives. This method addresses a specific OBD requirement that the boost control system must not degrade in response time if the level of degradation could affect emissions results, and to set a malfunction indicator light (MIL) once certain regulatory procedures are executed by the PCM diagnostic system. Further, the slow boost response fault is not limited to any particular cause.
As one example, responsive to a change in operator torque demand, a desired boost pressure may be determined, and processed via application of each of a delay and lag filter that has a response time based on a time constant parameter. One or more boost actuators, such as an exhaust VGT or equivalent capability waste-gate, may be adjusted based on the desired boost pressure. For example, responsive to an increase in desired boost pressure, the waste-gate opening may be decreased. In parallel, a pressure deviation of the expected (filtered processed form of desired) boost pressure relative to the actual pressure may be checked for lying outside of a range defined by upper and lower boundaries. If qualifying vehicle operating conditions are also met, such as if a minimum vehicle run time has elapsed, a minimum manifold pressure is available, and a minimum engine temperature is maintained, an engine controller may track the response time of the boost pressure deviation outside of the allowed boundaries. That is, the controller may measure a duration elapsed when the expected and actual pressure differ enough to lie outside the upper or lower boundaries (for example, a duration over which the pressure deviation stays above a threshold deviation level). A disturbance that altered the boost pressure, though the command was fixed, also results in a deviation error that can be evaluated the same way. The qualifying vehicle operating conditions as well as the upper and lower bounds of the pressure deviation may ensure that departing from the boundary conditions are confirmed where the response time can be reliably used for assessing the boost pressure control system response time. If the measured response time is higher than a threshold duration, such as higher than a known nominal system's maximum response time for qualified conditions, the controller may infer boost degradation to inform both the required regulatory OBD monitor and alert the operator of degraded operation.
In this way, by evaluating the response time of a boost control system to a pressure deviation, not specifically dependent on the command or position of the boost actuators (such as waste-gate), boost pressure response degradation may be separated from normal response that was intended to be slow. Requiring that the deviation exceed a boundary ensures that an elevated control response occurs that should trigger a fast correction (and if not fast is likely a fault) and the purpose of using defined upper and lower boundaries is that differences in response time to boost pressure overshoot versus undershoot may be better accounted for since the control system may be tuned to react to these differently, reducing the likelihood of erroneous faulted or un-faulted indications. By confirming qualifying vehicle operating conditions, the response time can be compared only during conditions where the results are reliable. In particular, idle and near-idle operating conditions can be avoided, where response times are unreliable. By confirming that operating conditions match those of a complete test run, the results of the boost monitor can be relied on with a higher confidence factor. By performing the test non-intrusively, adequate detection can be provided without disrupting vehicle drivability and emissions. In addition, the monitor may be run without necessitating complex and computation intensive modeling. Overall, boost control may be better diagnosed and timely addressed, improving boosted engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.